The present invention relates to a method of operating a wind turbine. Further, the present invention relates to a system of controlling operation of a wind turbine. Finally, the present invention relates to a wind turbine.
In recent years, it has become an important aspect of grid codes to properly handle overvoltage occurring on power grids to which the wind turbines are connected. That is, wind turbines should be capable of staying connected to the power grids or ride through even in the case of overvoltage. Wind turbines being capable of staying connected to the power grids during overvoltage may be called overvoltage/high voltage ride through capable wind turbines.
Overvoltages (OV) are gaining attention from wind farm owners and wind turbine manufacturers due to the large amount of sensitive power electronics in wind turbines with frequency converters. The transmission system operators are also interested in this phenomenon, since situations in the grid connection system of offshore wind farms have occurred where the insulation systems have been stressed in situations never experienced before. Overvoltages as high as 2 p.u. have been observed. In these events, the main-circuit breaker tripped the wind farm cable at the on-land connection point and left the wind farm in isolated operation with the cable and the wind farm transformer. Although such events are rare, this represents a risk of damaging the equipment. The Danish transmission system operator thus performed investigations of such OV in connection with the planning of new offshore wind farms. These investigations have shown that the OV levels are influenced by many parameters, including operational characteristics of the Wind Turbine Generators (WTG) prior to the disconnection, protection systems, control and the accuracy of the representation of the cable and the transformers in the relevant frequency range.
According to the grid code studies, a voltage up to 2.0 times the nominal grid voltage can be applied at the WTG terminals. For example, in Australia, the temporary OV can be up to 1.6 pu and in Canada-Manitoba the temporary OV can be up to 2.0 pu.
In W. Sweet, “Danish Wind Turbines Take Unfortunate Turn”, IEEE Spectrum, vol. 41, no. 11, pp. 30, 2004 it was reported that on the west coast of Denmark an offshore wind farm called Horns Rev 1 connected by an undersea cable experienced over-voltages (OV) as high as 2 p.u. (per unit) when the main-circuit breaker tripped at the on-land connection point and left the wind farm in isolated operation with the cable and the wind farm transformer.
In W. Wiechowski, J. C. Hygebjerg and P. Børre Eriksen,—Higher Frequency Performance of AC Cable Connections of Offshore□. 7th int. Conf. on Large Scale Integration of Wind Power and on Transmission Networks for Offshore Wind Farms, pp. 211-217, 2008 it was reported the Danish transmission system operator, Energinet.dk, performed investigations of such overvoltage in connection with the planning of the new offshore wind farm Horns Rev 2. These investigations have shown that the overvoltage levels are influenced by many parameters, including operational characteristics of the wind turbine generator (WTG) prior to the disconnection, protection systems, control and the accuracy of the representation of the cable and the transformers in the relevant frequency range.
It is desirable to provide a method to handle overvoltage for wind turbines which are highly effective and easy to implement.